The overall goal of Core A is to provide molecular biology expertise and support for the Program Project by closely interacting with all three Projects and Cores. Core A will construct novel simian-human immunodeficiency virus (SHIV) strains encoding R5 env genes of primary HIV clade C strains (termed SHIVenvC's);isolate, characterize, and sequence env genes from SHIVenvC-infected monkeys during disease progression, and perform diagnostic PCR to assess viral loads. The Specific Aims are: 1. To insert env genes derived from Zambian infants with rapidly progressive HIV clade C disease into multiply engineered SIVmac239-derived backbones to increase the replicative capacity of the chimeric SHIVenvC strains and to accelerate disease progression in primates. 2. To construct a fully heterologous SHIV based upon an SIV backbone that differs from SIVmac239, such as SIVsmE543-3, and by inserting an R5 HIV clade C env gene that differs from those used to construct the current SHIVenvC isolates. 3. To generate and characterize HIV clade C env mutants and to work closely with Project 1 to assess the molecular evolution of env in SHIVenvC-infected rhesus monkeys during disease progression. 4. To ensure that all experimental monkeys are free of other retrovirus infections prior to enrollment. A highly sensitive DMA PCR assay will be used to monitor for simian retrovirus type D (SRV/D) or simian Ivmphotropic virus type 1 (STLV-1). 5. To monitor all experimental animals in Core C by real-time RT-PCR and DNA PCR for plasma viral RNA or proviral DMA loads, respectively.